The operation principle of inkjet printing technology is printing precisely ink droplets into each pixel through a printing ejection head, which requires that a volume and a landing location of each ink droplet are very accurate. If the ink droplets are not of a uniform volume, a thickness of a film is not uniform, thus leading to linear stripes during displaying.
A pressure at the printing ejection head relates to a height of a liquid level of ink in a liquid supply tank and an air pressure in the liquid supply tank. The height of the liquid level is reduced gradually during a printing process, thus causing gradual decrease of the pressure at the printing ejection head and gradual decrease of the volume of the ink droplet ejected from the printing ejection head. Hence, the volume of an ink droplet ejected initially greatly differs from the volume of an ink droplet ejected finally, thereby causing nonuniform printing.